Powershift transmissions in which a change in ratio or a change in gear often results as an overlapping shift with direct torque transfer between two shifting elements, but also double-clutch transmissions, require accurate regulation. For example, in an overlapping shift, the moment of torque transfer must be very precisely determined according to the driving situation and load so as to satisfy the requirements of riding comfort and driving dynamics.
The shifting of a shifting element designed, for example, as a multi-disc clutch of a power transmission, from an actuated to an unactuated state results in the practice by displacement of a double-acting clutch piston by means of a hydraulic medium, the clutch piston defining, with one first surface, a clutch space pressurizable with hydraulic medium and, with a second surface, a rest space with a resetting device acting upon the piston. As a rule, the clutch is actuated or engaged against the force of a spring, mainly a cup spring, or against a counterpressure always acting upon the clutch piston on the side of the reset space.
In such a structure, the needed shifting pressure has to be increased by the power of the counterpressure or of the spring tension thus reducing the available control range of a pressure pre-control valve or control valve customarily used as actuator.
In the typical case of use of a clutch in a powershift transmission, this means that approximately 15% of the pressure power needed to ensure the transmitting capacity of the clutch is supported on a housing via a spring in the reset space and is not available for the transmitting capacity. This reduction of the utilizable control range of the actuator is especially problematic in an area of low pressure in which a pressure regulator usually shows a very flat behavior and to ensure a certain pressure power has to be substantially more heavily supplied with current than is the case in areas of higher pressure.
Particularly problematic are shifts to gears with a higher power ratio, since, on one hand, very high torques appear in the shifting elements and, on the other, high requirements are placed on the shifting quality due to the low speed.
This results in a conflict of interest in the layout of the shifting elements, since a high transmitting capacity requires a large clutch piston which disadvantageously is very sensitive to interruptions and needs a strong recoil spring.
However, a strong recoil spring has the disadvantages of great spring tension tolerance, great hysteresis and strong setting property together with the disadvantages typical in the use of springs. To the latter belongs a larger installation space, as a rule, the requirement that the aging of the spring be taken into account in the new layout and high expenses for adaptations in the transmission software which take into account the tolerance of the spring in the new state and its wear characteristics.
In addition, a strong recoil spring has to be compensated by an adequate layout of the control valve which results in an impaired ratio of the transmitted torque in correlation to the expenditure amount and thus to an impairment of the degree of effectiveness. Problematic here, on one hand, is the comparatively strong excitation pulse required to overcome the reaction inertia of the clutch piston and the fact that quantity of the output signal of the current of the control valve can be delivered only in discrete steps which, in as data amount imaginably to be processed, can result in a jerky movement of the clutch piston.
To implement a high transmitting capacity also requires a large friction surface to be distributed among a large number of friction elements or discs subject to a limited admissible contact pressure. But a large number of friction elements disadvantageously implies a great air play and thus a long travel stroke.
In the solutions known from the practice where the clutch is shifting or the clutch piston adjusted against a cropping out counterpressure, there is, likewise, the disadvantage of the reduction of the control range available and of the impairment of the ratio of the control pressure to be applied relative to the transmissible torque of the clutch.
The problem on which this invention is based is to provide a device for control of a hydraulically actuatable shifting element of a motor vehicle transmission, especially of a powershift transmission, with which the above described disadvantages are overcome, wherein a control of the shifting element is specially to be implemented in which the required spring tension of a recoil spring can be reduced, as much as possible and, at the same time, the ratio of a transmitted torque to a quantity of an output signal or a control current with low torque is increased without reducing the transmitting capacity.
According to the invention, the problem is solved with a device for control of a hydraulically actuatable shifting element of a motor vehicle, that is, with a clutch piston which defines, with one first surface, a hydraulically pressurizable piston space and, with one second surface of different size, a hydraulically pressurizable reset space, and having a slide valve system comprising a first clutch valve associated with the clutch space, a second clutch valve associated with the reset space and a holding valve associated with the reset space, which valves are displaceable according to a control pressure adjusted by a pressure adjuster, a change between a pressurization of the clutch piston, on the side of the clutch space and on the side of the reset space, being carried out as a control function in a manner such that the clutch piston is pressurized on its surface facing the reset space, in an unshifted state of the shifting element and, in a shifted state of the shifting element, is unloaded and both surfaces of the clutch piston, when the shifting element is engaged, are pressurized with at least approximately the same pressure up to a pre-defined pressure-adjuster control pressure.